behavior of ServerAliveInterval with ssh connection

divyashree · 10-21-2016, 09:12 PM

Hi experts,

Using ssh I am logging to another system and executing scripts there that creates new machines, and do some setups. It takes around 7-8 hours. So what happened the ssh connection keeps dropping and I always get timeout with unsuccessful execution of the script.

So now I am using this argument along with ssh connection:

Code:

"ssh -o ServerAliveInterval=60 user@host ...."

This ssh is invoked multiple times. The problem is after few ssh connection, I am getting error :

Quote:

too many logins of user

and the after ssh connections are getting closed after successful logins.

So is it the behavior of the ServerAliveInterval, that keeps the ssh user login session in remote machine alive even after ssh work is over and that's why my further logins are disconnected ?

Turbocapitalist · 10-22-2016, 02:31 AM

ServerAliveInterval, used with ServerAliveCountMax, send a message over the encrypted channel and then officially disconnect if the limits for lack of response are exceeded. See the manual page of ssh_config for more. It won't produce any error message other than indicating that the connection is dropped. Maybe ServerAliveCountMax needs to be increased.

Regardless, if your connection is likely to break during a long task, I'd work with a terminal multiplexer like "tmux" or "screen". When you log in the first time, start "tmux" then start your task. If you get disconnected, log in again and reconnect with "tmux a".

The error "too many logins of user" is something else that is in the way and not SSH itself, at least it's not Open SSH server. Does your server have some kind of rate limiting like "sshguard" or "fail2ban" installed?

divyashree · 10-24-2016, 12:41 AM

Thanks Turbocapitalist

Quote:

Originally Posted by Turbocapitalist

ServerAliveInterval, used with ServerAliveCountMax, send a message over the encrypted channel and then officially disconnect if the limits for lack of response are exceeded.

So if I am not using ServerAliveCountMax along with ServerAliveInterval, will the sessions stay alive forever ?

Turbocapitalist · 10-24-2016, 04:10 AM

Quote:

Originally Posted by divyashree

So if I am not using ServerAliveCountMax along with ServerAliveInterval, will the sessions stay alive forever ?

If the TCP connection is broken or unreliable, the SSH connection will eventually hang in practice. If you have ServerAliveCountMax and ServerAliveInterval set, then at least you know when the connection is no longer there in practice. If you use a terminal multiplexer like "tmux" then you can recover seamlessly. You'll need to use keys and a key agent for optimal performance.

Code:

while ! ssh -i ~/.ssh/some_key_rsa -t server.example.com 'tmux a || tmux'; do sleep 2; done

A similar trick can be done with "screen" if you do not have "tmux" and cannot install it.

Habitual · 10-24-2016, 04:45 AM

Quote:

Originally Posted by divyashree

This ssh is invoked multiple times. The problem is after few ssh connection, I am getting error

How many connections you need to the server?
Try just one or two sessions and see if the situation persists.